Color copying apparatuses have been improved in printing performance, and this has made it possible to reproduce a document image in form of a printed image of highly accuracy. In such circumstances, it has been demanded that the color copying apparatuses have advanced prevention functions to take countermeasures against an act of counterfeiting a printing-prohibited image, such a banknote, marketable securities, a confidential document, or the like, by printing the printing-prohibited image that has been read out by a scanner or captured by a digital camera or a camera built in a mobile phone.
Copying prohibition techniques have been employed in copying apparatuses, and a conventionally known form of the technique is described as follows. Image data (readout image data) are obtained by reading out, with an image readout apparatus, a document image printed on a recording medium. Subsequently, the image data thus obtained are compared with image data (registered image data) of a specific image which are registered, as a copying-prohibited target, in a copying apparatus in advance. Then, whether printing of the readout image data should be executed or not is determined based on a result of the comparison. In the technique, furthermore, in a case where it is determined that no printing of the readout image data should be executed, printing of the readout image data is stopped simply, or the readout image data are replaced by different image data so that the different image data thus replacing the readout image data are printed.
Techniques of Patent Literatures 1 to 3 have been known as concrete examples of the above type of the conventional technique. In the technique of Patent Literature 1, (i) a specific pattern for preventing image copying is formed in a document surface, (ii) when an image on the document is read out, a detecting section detects the specific pattern out from image data, and (iii) the detecting section outputs a detection signal so as to cause a copying function of a copying apparatus to be stopped.
Patent Literature 2 discloses a technique in which an image processing apparatus includes (i) a binarization processing section including first and second color-extracting blocks, (ii) an OR circuit, (iii) a storage apparatus, (iv) a mark detecting section, (v) a document cutout section, and (vi) a matching section. In the image processing apparatus, the first color-extracting block performs a black binarization process for input image data, and the second color-extracting block performs, independently from the first color-extracting block that performs the black binarization process, a red binarization process for the input image data. Two sets of data thus processed independently from each other are converted to single data by the OR circuit, and stored in the storage apparatus. Then, the mark detecting section detects a square-shaped mark out from the input image data by using a template. Subsequently, the image cutout section cuts out a mark surrounded by the square-shaped mark thus detected, and then, the matching section evaluates similarity between the mark thus cut out and a copying-prohibited determination mark (specific mark). After this, a signal indicative of a result of this is outputted.
Patent Literature 3 discloses a technique in which, in an image processing apparatus, (i) a first color-spatial similarity evaluating circuit determines similarity between an image signal for an input document image and color distribution information on a specific image stored in a ROM, (ii) the number of pixels similar between the inputted document image and the specific image is counted based on the similarity thus evaluated, and (iii) similarity between the input document image and the specific image is evaluated based on the number of the pixels thus counted.